PARIS (Reuters) - Roger Federer made a comfortable start to his quest for a 100th title, despatching Fabio Fognini in straight sets to ease into the quarter-finals at the Paris Masters on Thursday.

The 37-year-old Swiss had only made a late decision to compete in Bercy after winning his 99th title in his home town Basel on Sunday but, despite a touch of raggedness, was sharp enough to outclass the Italian 6-4 6-3 in his opening match.

Federer's procession came after Novak Djokovic celebrated his return to the summit of world tennis with an even easier stroll into the last eight after Damir Dzumhur had to retire injured in the second set of their third round clash.

Federer's return to Bercy for the first time in three years delighted his vast Parisian fan club.

The Swiss was not at his regal best against the flamboyant Fognini, who calls Federer "the king of tennis", but then he did not need to be with his opponent having never taken a set off him in three previous encounters.

Federer broke twice in the first five games, prompting the explosive Fognini to hurl down his racket in disgust, but the Italian did claw a break back before the 2011 Paris champion made slightly heavy weather of serving out the set.

After some uneven tennis in the second set, Federer completed the routine job with a fourth break of serve, this time to love as Fognini finished with a double fault, to wrap up a 73-minute win.

WINNING STREAK

Djokovic, who knew on Wednesday he would be world number one next week after Rafa Nadal's withdrawal from the tournament through injury, was leading 6-1 2-1 when Bosnian Dzhumur succumbed to a back problem that had flared up in the first set.

Djokovic extended his winning streak to 20 straight matches as he maintained the remarkable spell of form that helped him rise from number 22 in June to the top of the game again.

"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement. And, of course, I'm very, very happy and proud about it," the Serb told reporters.

Dzumhur suffered a tweak to his lower back when 4-1 down in the opening set. Clearly in pain, he battled on in an 11-minute sixth game but was eventually broken for a second time and, though he continued after treatment in a time-out and taking pain-killers, there was no way back on Bercy's centre court.

Djokovic, looking much sharper than in his opening match against Joao Sousa, took the set in 39 minutes and, once he had earned an inevitable break for 2-1, consoled the Bosnian, who told him at the net that he was in no shape to continue.

Seeking a record-equalling 33rd Masters and record-extending fifth Paris title, Djokovic will face a much tougher test in the last eight against Marin Cilic, who looked in strikingly good form when beating Grigor Dimitrov 7-6(5) 6-4.

Cilic, who moved closer to securing a place at the ATP Finals in London, beat Djokovic here at the same stage two years ago and also in this year's Queen's final before Wimbledon.

ISNER BLOW

John Isner's hopes of making the London finals received a potentially fatal blow when he was beaten 6-4 6-7(9) 7-6(8) by rising Russian Karen Khachanov.

The giant American saved four match points before the 22-year-old Khachanov, who had already staved off two himself, finally threaded a backhand winner past the net-rushing Isner to win after nearly 2-1/2 hours of massive hitting.

In a high-quality contest between two other players seeking a London berth, Dominic Thiem, back in the city where he reached the French Open final, ended Borna Coric's slim hopes while boosting his own with a 6-7(3) 6-2 7-5 win.

The Austrian will next face a rejuvenated Jack Sock after the defending champion, who has endured a dramatic slump in the year since, outplayed Tunisian lucky loser Malek Jaziri 6-0 6-4 to win back-to-back matches for the first time in 2018.